1. Field of the Invention
The present invention relates to a sheet processing apparatus that is connected to an image forming apparatus or a sheet processing apparatus in which the image forming apparatus is built in, such as a copying machine, a printer, a facsimile machine, and the like for processing a sheet discharged from a sheet discharging section thereof.
2. Discussion of the Background
Conventionally, in a case of a sheet processing apparatus which is connected to an image forming apparatus, or a sheet processing apparatus in which the image forming apparatus is built in, which receives a copied sheet from the sheet discharging section of the image forming apparatus, and which discharges the bundle of sheets towards a sheet discharging tray after stapling the sheets stacked on a staple tray as a bundle of a predetermined number of the sheets, tail end portions of the stacked sheets are vertically aligned by a rear end fence, and the side end portions of the sheets are laterally aligned by a pair of jogger fences. The sheet processing apparatus provided with the aforementioned construction has a staple unit that moves to a predetermined position of the tail end portion of the sheet, and staples the sheets at one position or two positions, as is well known.
On the other hand, the staple tray that stacks the sheets is disposed in an inclined state so that the sheet processing apparatus can be prevented from taking too much horizontal space. In the so inclined staple tray, the weight of the sheets is divided into a stacking direction (thickness direction of the sheet) and a conveying direction (longitudinal direction) of the sheet. The weight of the sheets subtracted from the thickness direction of the sheets is applied to the longitudinal direction of the sheets. When the weight is applied to the longitudinal direction, the sheets bend toward the rear end fence and vertical sheet alignment is thereby deteriorated.
Further, in a sheet processing apparatus capable of accepting a large number of sheets to be vertically aligned and stapled, the clearance in a thickness direction of the sheets within the rear end fence and a pair of jogger fences is increased when stacking a small number of sheets. Accordingly, since the sheets are not stacked along a base face (sheet stacking face) of the staple tray, a curl of the sheets (a state where the sheets separate from the staple tray at a tail end portion of the sheets) or a bending down of the sheets (see FIG. 21) tends to occur and the sheets are not accurately aligned.
FIG. 21 is an illustration showing the sheets bending down in a rear end fence and FIG. 22 is a perspective view showing a bundle of sheets stapled in a deviated state at a tail end portion thereof. A staple unit 11 includes a fixed part 11a and a movable part 11b that contacts and separates from the fixed part 11a. The staple unit 11 staples the sheets with a staple needle contained in the fixed part 11a or the movable part 11b by pressing the stacked sheets nipped between the fixed part 11a and the movable part 11b. In the case of stapling flexible sheets, or in the case of stapling sheets in a back-curled state as shown in FIG. 21, the sheets are stacked at a position deviated from the fixed part of the staple unit 11, which is a side of a rear end fence 19 at an end face of the sheets. If stapling is executed in this state, the sheets are stapled while pressed by the movable part of the staple unit 11 and accordingly the end face of the stapled sheets is deviated in an inclined state as shown in FIG. 22, and the vertical alignment of the sheets is deteriorated.
Furthermore, the sheets stacked at the rear end fence 19 of the sheet processing apparatus are guided by a bent guide portion formed by bending the rear end face, after being discharged onto the staple tray from the image forming apparatus and passing through the sheet discharging roller 6. The distance between the sheet discharging roller 6 and the rear end fence 19 is determined by the size of the staple unit 11; the larger the staple unit 11, the farther the distance between the sheet discharging roller and an upper edge of the rear end fence.
Furthermore, when the staple unit 11 is provided with an askew staple function, a larger space is required so that the staple unit 11 can pivot on the basis of the stapling position. Therefore, a distance between the sheet discharging roller 6 and the upper end portion of the rear end fence 19 is made longer. If the distance is made longer, the height of the bent part of the rear end fence should be larger so that the sheet can stably be guided, and if the height of the bent part of the rear end fence is made larger, the tip end of the bent part of the rear end fence cannot keep positional accuracy. Furthermore, the rear end fence 19 does not have a width as wide as the entire width of the sheet conveying path. Accordingly, the farther the distance between the sheet discharging roller 6 and the rear end fence 19 becomes, the harder it becomes to guide the sheet.
Moreover, since the staple tray is inclined, the effect of gravity in stacking the sheets on the staple tray becomes small, and the sheets are not tightly stacked. In other words, stacking efficiency deteriorates.